Network Working Group G. H. Mealy
Request for Comments: 195 HARV
NIC 7140 16 July, 1971
Categories: D.4, D.7
Data Computers -- Data Descriptions and Access Language
According to the minutes of the NWG meeting in May (RFC164), it
appears that a unified approach to Network data management is being
proposed to CCA. The purpose of this paper is to discuss some of the
problems involved and to suggest possible avenues of approach toward
their resolution. Parenthetically, I believe that a non-unified
approach leads to even worse problems.
My main remarks are predicated on a few assumptions and their
consequences. Since some or all may turn out to be wrong, it seems
appropriate to state them eXPlicitly. The steps in the arguments
leading from the assumptions to their consequences may appear to be
(and in fact may be) less than obvious. They are all of a piece,
however, and revolve around the necessity for doing business with a
number of dissimilar HOST systems while attempting to make it
unnecessary for an individual user or user program to know the
details of data file organization and representation. Given this as
an objective, I believe that the arguments are quite direct.
Assumptions
------------
1. We face the usual set of naming, cataloging, protection,
backup, etc. problems.
(I say this only to dismiss the subject as far as the following
is concerned. In my estimation, these problems and feasible
solutions are reasonably well understood; our real problem in
this area is in reaching agreement on specifics while leaving
sufficient ratholes for future expansion).
2. Files stored will contain arbitrarily complex data objects.
3. The organization of any file (that is, the way its strUCture is
mapped into physical storage by the data computer) will
normally be unknown by the user.
4. Data items in files may be stored in arbitrary representations
(e.g., those of the originating user's HOST rather than that of
the data computer or other "standard" representation).
5. Access to a file will normally be to some subset of it. (I.e.,
the unit for transmission will usually be part of a file rather
than the whole file, and access will not necessarily be
sequential).
Consequences
------------
1. A method of data description significantly more powerful than
now commonly available (as with COBOL or PL/I) is required.
The descriptions must be stored with the files. Data item
representations and storage organizations must be describable.
2. The data computer must offer a "data reconfiguration service",
based on use of the data descriptions.
3. A representation and organization-independent level of
discourse must be made available for controlling access.
Data Description
-----------------
As it happens, the descriptive facilities in ELl (References 1 and 2)
are almost adequate as they stand. ELl is an extensible language --
the compiler and interpreter for ELl are principal components of a
system implemented on the PDP-lO at Harvard -- which allows the
definition of arbitrary data structures in terms of a few primitive
data types (BOOL, CHAR, INT, REAL, SYMBOL, MODE, FORM, and ROUTINE).
These data types are of the sort I called "generic" in Reference 3.
To the EL1 implementation on the PDP-10, say, we would have to add
methods to describe a specific representation of INT, etc. and
primitive routines to convert between specific representations.
In the ECL system (in which EL1 is embedded), there is no rigid
distinction between compile time and run time. In particular, if the
arguments and free variables of a routine are evaluable at compile
time, then the routine is evaluated and the value replaces the call.
More generally, arbitrarily large amounts of a routine being compiled
may collapse into values. As far as the data computer is concerned,
this offers the possibility of producing tailor-made data
reconfiguration programs, taking maximum advantage of the data
descriptions at compile time rather than using a strictly
interpretative mode of operation.
Access Language
---------------
Here, I am on less firm ground. I will suggest, however, that some
of the ideas of Sattley, et al (Reference 4) deserve consideration.
I will quote from the Reference:
"... Our proposal is a language for describing the transferable
features of files, in which conventional programming languages (e.g.,
FORTRAN, ALGOL, etc.,) can be embedded, and from which the
information necessary to optimize the use of secondary storage can be
easily abstracted. This language defines our abstract model of
secondary storage in the same way that FORTRAN defined an abstract
machine. This language should have (at least) the following
features:
1. File declarations name the file and the elements in the file,
and specify the range of forms that the elements can take.
Each file has precisely one named element. Each file includes
the (maximum) size (in number of elements) of the file.
2. Subsets of files can be created by means of grouping
declarations. Such subsets can be nested.
3. Subsets of files can be named by means of naming declarations.
Such declarations can also name individual elements of the
file. Some form of implicit naming, allowing language
constructs such as GET ANOTHER TRIPLE, is included.
4. Members of a set (i.e., elements in a subset or file, subsets
in a containing subset or file) can be ordered by order
declarations. Some form of arbitrary but fixed ordering,
allowing language constructs such as GET NEXT TRIPLE, is
included.
5. The portions of a file transacted with at a point of access is
declared. The size of this portion can be expressed in
absolute or relative terms.
6. At each point of access to secondary storage, an environment is
described (or referenced) which contains those declarations of
types (l)-(5) necessary to define the transaction with
secondary.
A language with the above features makes no mention of hardware
devices, but it provides the programmer with the means of defining
the algorithm-dependent features of his files so that those files
might be transferred efficiently from machine to machine".
In the Sattley, et al study, the notion was that a compiler would
take the source program and actually compile the hardware-dependent
file accessing code. In our environment, we are concerned with
control commands to the data computer (e.g., GET NEXT WALDO) and
supplying the data computer with enough information to define a
WALDO. The basic functions would seem to be the same, in either
case, albeit implemented rather differently.
References
1. Wegbreit, B. The Treatment of Data Types in EL1. Technical
Report, Division of Engineering and Applied Physics, Harvard
University, Cambridge, Massachusetts, May 1971.
2. Wegbreit, B. The ECL Programming System. Technical Report,
Division of Engineering and Applied Physics, Harvard University,
Cambridge, Massachusetts, April 1971.
3. Mealy, G. H. Another Look at Data. AFIPS Conference Proceedings,
vol. 31, 1967 Fall Joint Computer Conference
4. Sattley, K., Millstein, R. and Warshall, S. On Program
Transferability. Report CA-7011-2411, Massachusetts Computer
Associates, Wakefield, Massachusetts, Movember 1970.
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